I sold mine after a month of use - was very disappointed with autofocus performance in low light. It was slow focusing or sometimes never at all in low light. During day it was an ok lens for a superzoom. Due to the fact that I shot a lot inside or at night, this was unacceptable to me. The stiff zoom in the middle I could deal with, autofocus issues - I could not. I've read many reviews by others in agreement with me re: slow slow focusing in low light. I think the kit lens is better.
I'd say the focusing speed is about mediocre, perfectly OK for me on a superzoom like this.
However, I wouldn't be so harsh on this lens regarding low light focusing. I'd say it's comparable to most lenses in this category. Do not expect this lens to have the same low light focusing performance as say an 85 1.8 or even 17-55 IS have, that'd be unreasonable.
I had the kit lens (18-55 IS) and the 55-250 IS before I bought this lens and they were about the same in low light regarding focusing ability. In low light, just like in good light, but especially in low light you need to find a decent contrast area to focus on.
Bottom line is, if you shoot a lot of pictures in very low light, you should not use this lens for that purpose, get a fast prime or a high end fast zoom, simple as that.
I have a 17-55 IS and when I know I need to shoot an indoor, low light event, I take that lens + a 430EX and not my 18-270. They can co-exist peacefully in my bag, the Tammy is my #1 vacation/walkabout lens. I rarely run into situations with my Tammy where it would hunt or refuse to focus in low light. If light is so low that I can't my Tammy to focus, then my pictures would probably come out crap anyway.
I think this lens is excellent for what it is, and perhaps the best in its category. But there are countless other threads dicsussing that.
Congrats on the new lens.
In fact, I'm just about to head out for a bike ride/walk in this 80 degree weather and taking my XTi + Tammy with me 